Adhesive bonding is a key process step in the fabrication or repair of many automotive and aircraft structural components. Adhesive bonding can eliminate the need for metal fasteners and their associated local stress concentrations. Most adhesive bonding for structural applications is initiated by application of heat. This is especially true in applications requiring rapid bonding, since adhesive curing is in general a temperature-dependent chemical reaction.
Some recent structural assemblies are composed of materials which melt at low temperatures, such as fiber-reinforced thermoplastic composites. Bonding such materials using heat-curable adhesives is difficult, because the high-temperature air typically used to heat the adhesive can warp or melt the composite. High temperature bonding of subassemblies composed of materials with different thermal expansion coefficients may cause residual thermal stress upon cooling the bonded assembly to room temperature. Composite assemblies with anisotropic coefficients of thermal expansion due to alignment of reinforcing fibers will warp when heated. Composites with complicated geometries, even those composed of isotropic materials, may warp upon heating, leading to debonding or cracking when cooled to room temperature. Some structural assemblies contain low density foam placed between composite shells to provide high stiffness and low weight. The foam can act as insulation material, preventing heat from reaching and curing adhesive bond lines.
In order to bond parts together successfully, particularly those with different coefficients of expansion, adhesive formulations must have a high degree of elastomeric character, which is typically not found in coating formulations. Therefore, in addition to being fully cured and solvent free, adhesives must meet requisite lap shear strength and extensibility values (i.e. high elongation to failure).
U.S. Pat. No. 2,688,133 issued to Brophy et al. on Feb. 2, 1954 discloses adhesives that are radiation curable.
U.S. Pat. No. 5,338,588 issued to Billiu on Aug. 16, 1994 discloses adhesives curable by electron-beam energy in a process for fabricating automotive composite assemblies. Radiation-curable adhesives, particularly electron-beam curable adhesives, are used in many such applications, since a cure may be effected with very little heating, and since electron-beams can penetrate through many materials, including foam, to reach the adhesive. Radiation-curable adhesives are also desirable because of their rapid rates of cure, compatibility with automatic dispensing equipment, and the environmental advantages of solvent-free systems. Generally, radiation-curable adhesives are cured using either ultraviolet energy in combination with a chemical photoinitiator, or electron-beam energy. However, most structural materials, including automotive and aircraft structural assemblies, require the deep penetrating ability of electron-beams as the materials they are formed from are opaque to ultraviolet light. Therefore, radiation-curable adhesives have not been useful in the assembly or repair of automotive or aircraft components, and they have been primarily limited to niche markets in the medical and electronics fields, due to high equipment costs as well as process and safety limitations. For example, the equipment used to generate electron- beams is considerably more costly than that required to generate ultraviolet light, and also requires radiation shielding for personnel safety, which typically consists of thick concrete walls and safety interlocks.
Typical accelerator and shielding costs and a description of the cost tradeoffs are given in the article by Goodman, et. al., "Advanced Electron-beam Curing Systems and Recent Composite Armored Vehicle Results," (Proceedings of The Journal of the Society for the Advancement of Material and Process Engineering (SAMPE) Vol.42, 1997 p.515-525). The relationships between electron-beam dose, energy, current and throughput are given in the review by Cleland ("High Power Electron Accelerators for Industrial Radiation Processing, in Radiation Processing of Polymers," A. Singh and J. Silverman, eds., Oxford University Press, N.Y. 1992, p.34-38).
In electron-beam curing, the system throughput, which is the amount of adhesively bonded material processed per unit time, depends on the rate at which a minimum dose can be delivered to the adhesive bond. Electrons penetrate through a depth of material proportional to their initial energy and inversely proportional to the material density. For beam energies exceeding the threshold needed to penetrate to the adhesive bond, the dose rate is nearly independent of energy and depends only on beam current. As described in the 1997 Proceedings of SAMPE article by Goodman, et. al., the cost of electron accelerator systems are primarily a function of accelerator power, (the product of electron gun voltage and current) and increase rapidly at powers above 10 kW. A process which can substitute lower power electron-beam equipment where higher power equipment was previously thought to be required, will significantly reduce equipment capital costs.
Conversely, if two electron-beam systems possess equal power, and both exceed the minimum energy to penetrate to the adhesive bond, the system with lower beam energy is more efficient, and will have a higher throughput. This is because less beam penetrates beyond the adhesive interface and is wasted in heating the interior of the part. Thus, a process which can substitute lower energy equipment of a given power, where higher energy equipment of equal power was thought to be required, will provide greater throughput, and therefore processing cost advantages.
As shown in the 1997 Proceedings of SAMPE article by Goodman et. al., the required shielding thickness (and therefore facility cost) increases with increasing electron-beam energy. Significant facility cost savings can be made if a process can substitute lower energy electron-beam equipment where higher energy equipment was previously thought to be required. Such a facility will also require less area, or allow larger parts to be processed in the same area, since a smaller fraction of the available space is taken up by the shielding.
Woods et al. disclose dual-cure adhesives that are rendered thermally curable by incorporating a thermal cure initiator capable of producing free radicals on heating, such as, for example, peroxides, azo compounds and disulfides. Curing of the adhesives typically requires exposure to radiation followed by heating in an oven or by directed heat lamps, typically to temperatures above 60.degree. C., and often above 100.degree. C., in order to activate the thermal cure initiators.
In "Structure-Property Behavior of Segmented Polyether-MDI-Butanediol Based Urethanes: Effect of Composition Ratio," by S. Abrouzahr et. al. in Polymer 23, p. 1077 (1982), polyurethanes are disclosed in which the hard pahse (with a T.sub.g above ambient) and the soft phase (with a T.sub.g below ambient) are well-formed regions which contribute to the elastomeric character at temperatures between the two T.sub.g s.
Radiation-sensitive phase-separated elastomeric adhesives are advantageous because they stretch when required to do so and return to their original dimensions upon release of a force. That is, mechanical hysteresis is minimal at appropriate extensions in these materials. In order to provide high elongation to failure values that are required for elastomeric adhesives, phase-separated polyurethane adhesives require well-defined domain structures including both a soft phase and a hard phase. The soft phase must be capable of rubbery behavior between the glass transition temperatures of the two phases, and not lead to extensive phase mixing on extension before break. In order to have soft and hard phases, polymer chain segments long enough to separate into well ordered regions must be present, both in the soft phase and in the hard phase, which typically requires a polyol having a molecular weight of at least 600. Polyols of insufficient molecular weight between cross-linking reactive groups will not be able to phase separate and behave elastomerically.
U.S. Pat. No. 4,342,793 issued to Skinner et al. on Aug. 3, 1982 disclose dual-cure coating compositions that are radiation and thermally curable, and include a radiation-sensitive reactive diluent, a chemical photoinitiator, and a thermally curable portion, reacted together to form an interpenetrating network. The thermal curing is effected with heat generated by a urethane reaction between an isocyanate and a saturated polyol. Both simultaneous and sequentially polymerized networks are present in the adhesive.
Therefore, a need exists for a radiation-curable phase-separated elastomeric adhesive that may be cured at room temperature.